Fluoropolymers are characterized by the fact that they are highly inert, paraffinic polymers in which some or all of the hydrogen have been replaced by fluorine. Fluoropolymers in general, and polytetrafluoroethylene (PTFE) in particular, have exhibited utility as materials for use in harsh chemical environments which may degrade many conventional polymeric materials. PTFE also has a useful range of temperature from as high as 260.degree. C. to as low as near -273.degree. C.
However PTFE exhibits poor mechanical properties, such as low tensile strength and low cold flow properties. In particular films of low porosity PTFE made by a skiving process in which solid PTFE films are shaved or split from a thicker article exhibit poor strength and flexibility and thus cannot be combined in a single structure with highly flexible materials such as neoprene rubber. These poor mechanical properties limit the use of PTFE in many situations.
PTFE may be produced in an expanded porous form as taught in U.S. Pat. No. 3,953,566. This material, expanded porous polytetrafluoroethylene (ePTFE), has a microstructure consisting of nodes interconnected by very small fibrils and is of higher strength than unexpanded PTFE while maintaining the chemical inertness and wide useful temperature range of PTFE.
However, ePTFE cannot be used as a barrier layer to chemicals since it can rapidly absorb through its pores liquids that have a low surface tension, e.g., less than 50 dynes/cm. A process that makes films, sheets or forms of various thickness out of densified ePTFE having substantially no porosity maintaining the high strength of the node and fibril structure of ePTFE would therefore have wide utility as a barrier to harsh chemicals.
Dense ePTFE structures are taught in U.S. Pat. No. 3,953,566 in which a platen press is used to densify a very thin sheet of ePTFE either with or without heat, however in practice using multiple layers results in the trapping of air within the expanded structure and also between layers so that high densities are not achieved in structures having thickness greater than the starting sheet. Also cold flow takes place in the press resulting in non uniformly shaped final parts. Dense EPTFE structures are also described in U.S. Pat. No 4,732,629 to Cooper et al., however the method used is not able to generate high densities in thick films and again air is trapped within the fine structure of the ePTFE plies resulting in low densities. U.S. Pat. No. 5,061,561 to Katayama discloses a method to make fibers with high density from ePTFE similar to that used in this invention, however, the method yields a material that is significantly different from that in this invention as evidenced by DSC peaks at 345.degree. C. and 380.degree. C. Furthermore the Katayama process is different and is applicable only to fine filaments and not to sheets or formed shapes.